The present invention relates generally to discrete multi-tone communication systems in which a central unit services a plurality of remote units. More specifically, it relates to methods for coordinating upstream communications from the remote units.
Discrete Multi-Tone (DMT) data transmission schemes have been shown to facilitate high performance data transmission. Among the benefits of DMT architectures is that they have high spectral efficiencies and can adaptively avoid various signal distortion and noise problems. Since they have very high data transmission capabilities, in most applications selection of a DMT data transmission scheme will provide plenty of room for the expansion of service as the demands on the data transmission system increase. Hence, discrete Multi-Tone technology has applications in a variety of data transmission environments. For example, the Alliance For Telecommunications Information Solutions (ATIS), which is a group accredited by the ANSI (American National Standard Institute) Standard Group, has finalized a discrete multi-tone based standard for the transmission of digital data over Asymmetric Digital Subscriber Lines (ADSL). The standard is intended primarily for transmitting video data over ordinary telephone lines, although it may be used in a variety of other applications as well. The North American Standard is referred to as the ANSI T1.413 ADSL Standard.
Transmission rates under the ADSL standard are intended to facilitate the transmission of information at rates of at least 6 million bits per second (i.e., 6+ Mbit/s) over twisted-pair phone lines. The standardized discrete multi-tone (DMT) system uses 256 xe2x80x9ctonesxe2x80x9d or xe2x80x9csub-channelsxe2x80x9d that are each 4.3125 kHz wide in the forward (downstream) direction. In the context of a phone system, the downstream direction is generally considered transmissions from the central office (typically owned by the telephone company) to a remote location that may be an end-user (i.e., a residence or business user). In other systems, the number of tones used may be widely varied. However when IFFT modulation is done, typical values for the number of available sub-channels (tones) are integer powers of two, as for example, 128, 256, 512, 1024 or 2048 sub-channels.
The Asymmetric Digital Subscriber Lines standard also contemplates the use of a reverse signal at a data rate in the range of 16 to 800 Kbit/s. The reverse signal corresponds to transmission in an upstream direction, as for example, from the remote location to the central office. Thus, the term Asymmetric Digital Subscriber Line comes from the fact that the data transmission rate is substantially higher in the forward direction than in the reverse direction. This is particularly useful in systems that are intended to transmit video programming or video conferencing information to a remote location over the telephone lines. By way of example, one potential use for the systems allows residential customers to obtain video information such as movies over the telephone lines or cable rather than having to rent video cassettes. Another potential use is in video conferencing.
The discrete multi-tone (DMT) transmission scheme has the potential for use in applications well beyond data transmissions over telephone lines. Indeed, DMT can be used in a variety of other digital subscriber access systems as well. For example, it may be used in cable based subscriber systems (which typically use coaxial cable) and wireless subscriber systems such as digital cellular TV. In cable systems, a single central unit (central modem) is typically used to distribute digital signals to more than one customer, which means more than one remote unit (remote modem). While all of the remote modems can reliably receive the same digital signals, the upstream transmissions must be coordinated to prevent confusion at the central modem as to the source of the upstream signals. In some existing cable systems (which do not use discrete multi-tone transmission schemes), each remote unit is given a dedicated frequency band over which it is to communicate with the central station. However, such an approach is inherently an inefficient use of transmission bandwidth and typically requires the use of analog filters to separate transmissions from the various remote units. Other existing cable systems use a single wide band for all remote units, which use time division multiple access (TDMA) to access the upstream channel. This approach is inefficient because of the lower total capacity of the single channel and because of the time required for the accessing process. Stationary digital cellular transmission systems face similar obstacles. The ability to access the channel on both a time- and frequency-divided basis would more efficiently utilize the transmission channel. However, the inherent multiplexing nature of DMT has previously restricted its application to point-to-point transmission because transmissions from different sources must be synchronized for the all-digital multiplexing to function properly.
ADSL applications have the potential for a similar problem, although it is typically more limited in nature. Specifically, a single line may service a plurality of drop points at a particular billing address (which may typically be a home or an office). That is, there may be several telephone xe2x80x9cjacksxe2x80x9d through which the user may wish to receive signals. To facilitate service to multiple locations (jacks) over a single line, the use of a master modem has been proposed to facilitate synchronization. However, this is perceived as being a relatively expensive and undesirable solution. Accordingly, it would be desirable to provide a mechanism in discrete multi-tone data transmission systems that facilitates the synchronization of signals from a plurality of remotes so that a central unit can coordinate and reliably interpret signals sent from the remotes.
Another feature of transmission systems currently utilized for communications from a remote unit to a central unit is that they either transmit data at a designated maximum rate (frequency-division multiplexing), or they transmit data in packets of a particular size (time-based multiplexing). They do not permit both. This limits the efficiency of the use of the transmission channels. Accordingly, it would be desirable to provide a mechanism through which when necessary, a remote unit can specify a desire to transmit at a particular data rate and when the data rate is not a concern, the remote unit may indicate that it desires to transmit a designated amount of information.
To achieve the foregoing and other objects and in accordance with the purpose of the present invention, a number of bi-directional data transmission systems that facilitate communications between a plurality of remote units and a central unit using a frame based discrete multi-carrier transmission scheme are disclosed. In each of the systems, frames transmitted from the plurality of remote units are synchronized at the central unit. A variety of novel modem arrangements and methods for coordinating communications between a plurality of remote units and a central unit to facilitate multi-point-to-point transmission are disclosed. The invention has application in a wide variety of data transmission schemes including Asymmetric Digital Subscriber Line systems that include the transmission of signals over twisted pair, fiber and/or hybrid telephone lines, cable systems that include the transmission of signals over a coaxial cable, and digital cellular television systems that include the transmission of radio signals.
In one embodiment, a discrete multi-tone data transmission system has a multiplicity of discrete subchannels including an overhead bus. In a method aspect, when a selected remote desires to initiate communications, it loop times its own clock with the clock of the central unit and then transmits a remote initiated synchronization signal to the central unit over a dedicated overhead subchannel or set of overhead subchannels in the overhead bus. The central unit responds with a centrally initiated synchronization signal that contains information indicative of a frame boundary phase shift required to better synchronize the selected first remote unit with other remote units that are currently communicating with the central unit. The remote responds by shifting the phase of the frames it outputs as indicated by the centrally initiated synchronization signal. The synchronization may be done in either an iterative manner or as a single step. This synchronizes the frame boundaries of the frames outputted by the selected remote unit with frame boundaries of frames output by the other remote units that are currently communicating with the central unit. The synchronization is arranged to occur such that the frame boundaries from the various remotes substantially coincide when they are received at the central unit.
In one embodiment of the invention the overhead bus includes two dedicated overhead subchannels and the remote initiated synchronization signal and the centrally initiated synchronization signal are transmitted over different overhead subchannels. In other embodiments a single or multiple dedicated overhead subchannels may be used. In some embodiments, the number of subchannels available to the selected remote unit for transmission of data to the central unit are dynamically allocated. Specific central and remote modem designs suitable for implementing such a system are also described.
In another aspect of the invention, synchronized quiet times are periodically provided in the upstream communication stream. The synchronized quiet times are used to handle a variety of overhead type functions such as initialization of new remote units, transmission channel quality checking and handling data transfer requests.
In one embodiment, a method of synchronizing frames transmitted from an initializing remote unit to the central unit with frames transmitted from other remote units to the central unit is described. In this embodiment, synchronized quiet times are periodically provided on the plurality of discrete sub-channels provided for upstream communications. When a remote unit is being initialized, it transmits a broad-band initialization signal to the central unit during a synchronized quiet time. The broad-band initialization signal includes a plurality of initialization signals transmitted over distinct sub-channels. In one preferred embodiment, the remote unit monitors downstream communication when it desires initialization and substantially synchronizes the frame boundary of the broad-band initialization signal with a frame timing marker carried in downstream signals received by the remote unit. The central unit receives the broad-band initialization signal and sends a synchronization signal back to the first remote unit. The synchronization signal includes information indicative of a frame boundary phase shift required to better synchronize frame boundaries of signals sent by the remote unit with frame boundaries of signals sent by other remote units that are in communication with the central unit. The remote unit then shifts the phase of the frames it outputs to facilitate synchronization.
The synchronized quiet time used in this embodiment has a period that is sufficiently long for a quiet period marker to be transmitted from the central unit to the remote unit that is furthest from the central unit, and an initialization signal returned from that furthest remote unit to the central unit all within the synchronized quiet time.
In another embodiment, a method of dynamically checking sub-carrier transmission quality from the remote units to the central unit is described. This facilitates the allocation of bandwidth to the remote units by the central unit. In this embodiment, training signals are transmitted from one of the remote units over the multiplicity of sub-channels provided for facilitating upstream communications during a selected synchronized quiet time. The training signals are monitored by the central unit which determines a set of channel characteristics indicative of the bit capacities of the various sub-channels to deliver signals from the selected remote. The central unit may then use the set of channel characteristics when determining which sub-channels to allocate to the selected remote unit for upstream communications.
In one preferred embodiment, the transmitting and monitoring steps may be repeated plurality of different remote units in order to determine channel characteristics for each of the different remote units. The different remote units are preferably arranged to transmit their respective training signals during different quiet times. The set of channel characteristics for each remote may be stored within a matrix of channel characteristics that contains information indicative of the channel capacities from each of the remote units to central unit. The channel characteristic information may then be used to facilitate the dynamic allocation of bandwidth to various remote units. In another preferred embodiment, the remote units only transmit their respective training signals in response to the reception of a retraining signal from the central unit. This facilitates control over the system.
In yet another embodiment of the invention, a method of informing the central unit of the transmission requirements of a remote unit is described. In this embodiment, a remote that wishes to initiate or change communications transmits a data request signal to the central unit at a time other than during a quiet time interval. The central unit then sends an authorization signal to the remote unit allocating a particular quiet time. The remote then transmits data request information over a plurality of the discrete sub-channels during the allocated quiet time. Knowing the remote unit""s requirements, the central unit allocates one or more sub-channels to the remote unit in response to the data request information.
In one preferred embodiment, the data request signal may indicate either a desire to transmit at a particular data rate or a desire to transmit a particular amount of information. In the former case, the central unit allocates sufficient sub-channels to the remote unit to facilitate transmission at a requested data rate that is specified in the data request information. In the latter case, the central unit allocates one or more sub-channels for an amount of time sufficient to transmit an amount information that is specified in the data request information.
In another preferred embodiment, the remote unit may transmit a defined data packet request signal after the data request information has been defined and transmitted. When this occurs, the central unit immediately allocates at least one sub-channel to the selected first remote unit in direct response to the defined data packet request.
In another preferred embodiment, the remote units monitor information provided in the downstream communications stream prior to transmitting a data request signal and only transmit the data request signal over sub-channels that are reported as not in use. In still another preferred embodiment, a first value of the data request signal is indicative of a data rate request, a second value of the data request signal is indicative of a data packet request and a third value of the data request signal is indicative of a defined data packet request. In such an arrangement, the data request signal may be as small as a two bit signal.
In yet another embodiment, each frame of the discrete multi-tone signal includes a multiplicity of symbols and each remote unit is assigned an associated symbol during which it may transmit its data request symbol. In this embodiment, the central unit determines the identity of a particular remote unit transmitting a data request signal based at least in part upon the symbol during which the data request signal is received.
It should be appreciated that the various embodiments may be used either standing alone or in combination with one or more of the others. The various described quiet times need not be of the same length and typically, the quiet times described in conjunction with the third embodiment would be more frequent than the other two.
In still another embodiment, a fast access transmission mode is provided. In this embodiment, a communication access request that includes a unique remote unit identifier is transmitted from the requesting remote to the central unit. The request is transmitted on at least one unused sub-channel using a modulation scheme that does not require equalization to decode at the central unit. The central unit then allocates the appropriate sub-channels to the requesting remote unit.
In further aspects of the invention, discrete multi-point transmitters and receivers capable of implementing the various methods are described. It should be appreciated that the various embodiments may be used either standing alone or in combination with one or more of the others. The described systems may be used regardless of whether the downstream signals are also discrete multi-carrier. In several preferred embodiments, the bidirectional data transmission system is a cable system that includes the transmission of signals over a coaxial cable, although other systems are contemplated as well.